X-ray detectors based on active readout matrices, e.g. of amorphous silicon (a-Si), in front of which a scintillator layer is located, have been known for some years. The impinging X radiation is converted into visible light in the scintillator layer, converted into electrical charge in light-sensitive pixel elements of the readout matrix and stored spatially resolved. Related technologies also use an active pixel matrix of amorphous silicon but combined with an X-ray converter (e.g. selenium) which converts the impinging X radiation directly into electrical charge. The stored charge is then read out electronically via an active switching element, converted into digital signals and forwarded to an electronic image processing system. Such X-ray detectors are frequently constructed as flat-panel detectors.
Many digital X-ray detectors exhibit a power dissipation which produces a steep increase in temperature or thermal fluctuations of the X-ray detector and its components. In general, temperature rises and fluctuations lead to offset patterns and sensitivity differences, especially in edge areas of the X-ray detector. This is mainly caused by temperature-related widening or shrinking of adhesive at contact points between the scintillator and the active matrix and leads to a reduced image quality of X-ray images at the edge areas. The width of this disturbance can comprise up to about 100 pixel readout elements from the scintillator edge inward in the direction of the scintillator center. This reduces the meaningfully useful width of an X-ray detector by about 3 cm.
Some X-ray detectors have active air or water cooling in order to prevent temperature rises or fluctuations. Such cooling is known, for example, from JP 11271456 A. It is also known to remove losses in the X-ray image quality, caused by thermal influences, from the image by subsequent electronic corrections.
In each row of the active matrix, sections are defined which are shielded from the X radiation. In general, this is implemented by the absence of a scintillator together with X-ray shielding (lead). The totality of these shielded sections is designated the so-called dark reference zone (DRZ). Measurement values detected in the dark reference zone, so-called dark values, are used for correcting the remaining sections of the respective rows, that is to say the useful area. This method is known as line noise correction (LNC).